Hey jt,
I'll go ahead and reply in here. The difference I noticed was on the SIMM pin 46 (left to right, with the notch side on the left). GttMFH2E says it is +5 volts, but the IIsi's dev note says it is A23 (GttMFH2E only goes up to A22, which is on pin 45). I really don't know if Apple actually connected that pin on the IIfx SIMM -- you'll have to check it. I'm assuming that hooking A23 to VCC would be a bad thing. The simple solution to make my new custom SIMMs compatible with both is to leave pin 46 not connected to anything. On my SIMM design, I had it connected to all the other +5V pins on the SIMM. I should have probably just left it unconnected. So if Apple connected that pin to anything on the IIfx SIMM, that could be causing your problem.
More likely though is that the IIsi's just too new for the IIfx's ROM code
I wonder if the reverse would work? (IIsi ROM in a IIfx)
I'll go ahead and reply in here. The difference I noticed was on the SIMM pin 46 (left to right, with the notch side on the left). GttMFH2E says it is +5 volts, but the IIsi's dev note says it is A23 (GttMFH2E only goes up to A22, which is on pin 45). I really don't know if Apple actually connected that pin on the IIfx SIMM -- you'll have to check it. I'm assuming that hooking A23 to VCC would be a bad thing. The simple solution to make my new custom SIMMs compatible with both is to leave pin 46 not connected to anything. On my SIMM design, I had it connected to all the other +5V pins on the SIMM. I should have probably just left it unconnected. So if Apple connected that pin to anything on the IIfx SIMM, that could be causing your problem.
More likely though is that the IIsi's just too new for the IIfx's ROM code
I wonder if the reverse would work? (IIsi ROM in a IIfx)